The Papova viruses, Polyoma and SV40, are oncogenic DNA viruses capable of both lytic infection and abortive infection resulting in cellular transformation. The murine cell line Balb/c 3T3 is susceptible to both types of infection by Papova viruses: lytic with polyoma and transformation with SV40. In any effort to understand the cellular functions required in these interactions, we have been isolating Balb/c 3T3 mutants which are temperature sensitive (ts) for growth. Among the several isolated, one (ts-2) has been extensively characterized thus far. Studies involving inhibitors and cell synchronization techniques suggest that the mutation in ts-2 directly effects DNA replication at the non-permissive temperature (C38). Polyoma virus will not replicate in ts-2 at C38.